Polymers are a mainstay of modern society, for example, widely used in fabricating textiles, upholstery, construction materials, various air, land or sea vehicles, and microelectronic devices and appliances. The inherent flammability of many polymers poses a significant threat, especially in enclosed or isolated spaces. Therefore, as synthetic polymers are used extensively in society as plastics, rubbers, and textiles, polymer flammability has been recognized as a safety hazard and remains an important challenge in polymer research.
Flame retardancy of polymers is often achieved by blending polymers with flame retardant additives, such as halocarbons, including polybrominated diphenyl ether (PBDE), phosphorus, organophosphates, and metal oxides. While small molecule flame retardant additives provide a convenient means for reducing flammability of polymers, these additives can compromise safety from environmental and health perspectives. Conventional flame retardants are small molecule additives that often leach out of the polymer during their use leading to a variety of serious health and environmental problems associated with toxicity and bioaccumulation. These concerns have led to an emphasis on non-halogenated flame retardants in recent years. However, non-halogenated flame retardant additives, such as alumina trihydrate, compromise the physical and mechanical properties of polymers when loaded at high levels.
An ideal low-flammable polymer would be halogen-free and possess high thermal stability, low heat of combustion, and a low combustion heat release rate (HRR), with minimal release of toxic fumes. Intrinsically fire-resistant polymers that undergo significant carbonization upon heating are highly desirable, as carbonaceous char formation effectively averts combustion by producing an insulating layer on the polymer surface. Such char formation can also be realized from composite materials in which an additive ultimately provides the desired char.
Aromatic polyesters prepared from bisphenols and phthalic acids are important high performance engineering thermoplastics. Conventional bisphenol A (BPA)-containing polyarylates are well-known and widely used, but exhibit higher-than-desired flammability (e.g., BPA-polyarylates have a heat release capacity (HRC) of about 400 Joules per gram-Kelvin (J/g-K)). Polyarylates containing 1,1-dichloro-2,2-bis(4-hydroxyphenyl)ethylene (bisphenol C, or BPC) are transparent and processable, and exhibit excellent mechanical and dielectric properties. BPC-containing polymers are well within the “ultra fire-resistant” category (HRC<100 J/g-K), with BPC-polyarylates and polycarbonates having reported HRC values of 21 and 29 J/g-K, respectively, and high char yields (50-55%). However, the presence of halogen in BPC-containing polymers, and the evolution of hydrogen chloride gas at elevated temperatures, remain concerns, thus limiting their adoption for scale-up and manufacturing as commodity materials.
BPC derivatives can be converted to the corresponding diphenylacetylene by loss of the chlorines, followed by phenyl migration. In BPC-containing polymers, this reaction represents a viable mechanism to char formation, in which the presence of chlorine sets up the rearrangement chemistry that leads to diphenylacetylene. In fact, diphenylacetylene-containing poly(aryl ether ketone)s showed heat release characteristics of similar magnitude to the corresponding BPC-containing polymers. However, these alkyne-containing aromatic polymers are prone to side-reactions and crosslinking even at moderately high temperatures, and have less-than-optimal processability and mechanical properties for many polymer materials applications. Accordingly, there is an ongoing search in the art for non-halogenated polymers or additives which promote charring or preclude combustion.
One approach has been the use of polyarylates incorporating a deoxybenzoin moiety, e.g., 4,4′-bishydroxydeoxybenzoin (BHDB), as a bisphenolic monomer. These polymers exhibited low combustion heat release rate and total heat of combustion, which is believed to arise from the thermally-induced conversion of BHDB to diphenylacetylene moieties that char by aromatization. See, K. A. Ellzey, T. Ranganathan, J. Zilberman, E. B. Coughlin, R. J. Farris, T. Emrick, Macromolecules 2006, 39, 3553. Pyrolysis combustion flow calorimetry (PCFC), an oxygen consumption technique for measuring heat release capacity (HRC), revealed exceptionally low HRC values for the BHDB-polyarylates (<100 J/g-K). See, R. N. Walters, M. Smith, and M. R. Nyden, International SAMPE Symposium and Exhibition 2005, 50, 1118. However, the rather low solubility of such polyarylate compounds limits their molecular weight and processibility.
Despite the ongoing interest in the field of flame retardant polymer materials, there remains a continuing need for synthetic strategies toward structurally and functionally diverse flame-retardant polymers that can overcome the above-described technical limitations.